1. Field of the Invention
The present invention relates to a semiconductor device comprising in its composition a thin film transistor (hereinafter, referred to as TFT) that uses a semiconductor thin film, and to a method of manufacturing the same. In this specification, the term xe2x80x9csemiconductor devicexe2x80x9d denotes devices in general which may function with utilization of semiconductor characteristics, and hence semiconductor circuits, electro-optical devices and electronic equipments in this specification all fall into the semiconductor device.
2. Description of the Related Art
In recent years, what have attracted attention is an active matrix type liquid crystal display device including a circuit constituted of a TFT that uses a polysilicon film. The device is designed to control on matrix-basis, by means of a plurality of pixels arranged in matrix, the electric field that is applied onto liquid crystal, thereby realizing image display of high definition.
In an active matrix type liquid crystal display device, an image is displayed by controlling with a pixel TFT formed for every pixel the voltage applied to liquid crystal. Today""s display device has a million or more pixels besides many other components, so that huge numbers of pixel TFTs have to be formed with good yield while holding down the variation in their characteristics.
The interface between an active layer and a gate insulating film may be given as one of parameters that influence the TFT characteristics most. When contaminated, this interface affects in an instant the TFT characteristics. Purifying the interface at which the active layer and other insulating film are brought into contact is therefore necessary to obtain good TFT characteristics.
The object of the present invention is to provide a method for forming a TFT exhibiting good electrical characteristics and, further, to provide a semiconductor device using such a TFT, the semiconductor device including a semiconductor circuit, an electro-optical device, an electronic equipment and the like.
The present invention is directed to a technique for purifying the interface between an active layer and an insulating film in a TFT. The gist of the present invention resides in that, on forming a TFT of bottom gate structure (typically, an inverted staggered structured), at least a channel formation region of a semiconductor film to be an active layer is not exposed to the air even once. With this construction, contamination at the interface of the active layer is prevented to realize a TFT exhibiting stable and good electrical characteristics.
According to one aspect of the present invention there is provided a method of manufacturing a semiconductor device that includes sequentially forming and layering, over a substrate having a gate wiring formed thereon, a gate insulating film, a semiconductor film and a protective film without exposing them to the air. The semiconductor film is then irradiated with ultraviolet light or infrared light to form a semiconductor film containing crystals. Then, an impurity region containing crystals is formed in the semiconductor film, while using as a mask the protective film. Next, the semiconductor film containing crystals is patterned into an island-like shape to form an active layer.
According to another aspect of the present invention, there is provided a method of manufacturing a semiconductor device that includes the additional step of etching the gate insulating film while using as a mask the active layer.
According to still another aspect of the present invention, there is provided a method of manufacturing a semiconductor device that includes forming a gate wiring on a substrate. Then, the gate wiring is oxidized to form a gate oxide film on the surface of the gate wiring. Next, a gate insulating film, a semiconductor film and a protective film are sequentially formed and layered on the substrate where the gate wiring is formed without exposing them to the air. Then, the semiconductor film is irradiated with ultraviolet light or infrared light to form a semiconductor film containing crystals. An impurity region is then formed in the semiconductor film containing crystals while using as a mask the protective film. The semiconductor film containing crystals is then patterned into an island-like shape to form an active layer.
According to yet another aspect of the present invention, there is provided a method of manufacturing a semiconductor device including the additional step of sequentially etching the gate insulating film and the gate oxide film while using as a mask the active layer.
In this specification, the term xe2x80x9csemiconductor film containing crystalsxe2x80x9d denotes a single crystal semiconductor film or a crystalline semiconductor film containing a crystal grain boundary (which includes a polycrystalline semiconductor film and a microcrystalline semiconductor film), and is used to clearly distinguish those films from a semiconductor film that is amorphous all over its area (an amorphous semiconductor film). Needless to say, what is mentioned simply as xe2x80x9csemiconductor filmxe2x80x9d includes an amorphous semiconductor film in addition to semiconductor films containing crystals. In the invention of the present application, the gate wiring may be formed solely of a tantalum film, or may be formed of a laminated film in which a tantalum film and a tantalum nitride film are layered. Of course, other conductive film or a silicon film may be layered on those films.
Further, it is effective to use as the gate insulating film a laminated film containing one layer of silicon nitride film (for example, a laminated film in which a silicon oxide film and a silicon nitride film are layered). This construction has an effect that the silicon nitride film prevents diffusion of contaminants from the substrate. It is also effective to use as the gate insulating film a laminated film containing one layer of BCB (benzocyclobutene) film. The BCB film, having very high flatness, may eliminate a level difference due to the gate wiring to enhance the flatness of all the thin films that are to be formed above the BCB film.
An another aspect of the above invention, the gate oxide film may be formed by any measure of a plasma oxidation method, a thermal oxidation method and an anodic oxidation method. The plasma oxidation method with which process is simple is particularly effective in terms of improvement in the throughput.